1. Field of the Invention
The present invention relates to the representation of digital image data, and in particular, to the binary or multilevel representation of images for display purposes.
2. Description of the Prior Art
The representation of continuous tone pictorial data in a binary format is a major concern as the need increases to transmit and display images. The binary representation of image information is desired in order to reduce the sensitivity to noise in transmission and storage or to enable the accurate depiction of continuous tone originals with binary media. In the general field of digital halftoning, there have been disclosed different methods to convert continuous tone to binary images in such a manner as to preserve the appearance of tonal gradation similar to the original image. For example, U.S. Pat. No. 4,625,222 to Bassetti et al. discloses a print enhancement control system for an electrostatic copying machine wherein control logic circuitry processes a set of image altering parameters to improve image production quality. These parameters, whose values are either predetermined, fixed or have been received from an exterior source, improve image quality (i.e., resolution) by modifying modulated gray signals.
U.S. Pat. No. 4,700,229 to Herrmann et al. discloses an image enhancement circuit which converts a low quality image signal into a high quality image signal by modifying the binary representation of a picture. Image enhancement is accomplished by multiplying a series of error difference signals by a series of weighting factors k(i) which produce a clearer image by improving picture resolution.
U.S. Pat. No. 4,672,463 to Tomohisa et al. discloses a method to improve image quality within an electrostatic reproduction machine wherein the sharpness of an image is improved based on the value of an image sharpness control parameter that has been calculated examining the copy quality of an original.
U.S. Pat. No. 4,709,250 to Takeuchi discloses an image forming apparatus which improves the halftone image quality of an original. The pulse width of a reference control signal controls and improves image quality in response to a detected image density signal.
U.S. Pat. No. 4,724,461 to Rushing discloses an image improving process control for an electrostatic copying machine which maintains high image quality by adjusting a set of process control parameters.
U.S. Pat. No. 4,256,401 to Fujimura et al. discloses an image density adjustment method wherein a predetermined image density level within an electrostatic copying machine is maintained at a standard density by varying a set of input control parameters.
U.S. Pat. No. 4,693,593 to Gerger discloses a method of improving the image quality by controlling a single process parameter in response to changes in sensitometric characteristics of an image transfer member.
These systems, although providing some degree of image improvement, generally do not provide the means to control the edge enhancement of regions within the image.
Other prior work, such as shown in the Proceedings of the Society for Information Display, Volume 17, 1976, includes a numerical screening algorithm, the Floyd and Steinberg error diffusion algorithm, as a coding technique for the binary encoding of greyscale image data. Modifications to the Floyd-Steinberg algorithm may, as shown by Billotet-Hoffman and Bryngdahl in the Proceedings of the Society for Information Display, Volume 24, 1983, include a varying threshold, a dither, instead of a fixed threshold. This visually decodable technique is presently utilized, for example, in computer displays where the resolution of the output device is limited. The adaptive nature of the Floyd-Steinberg algorithm automatically provides a sharp, edge-enhanced appearance which, while visually appealing, may not necessarily be desirable in the output image.
A difficulty with the Floyd-Steinberg error diffusion algorithm is that an inherent edge enhancement is built into the algorithm. Analysis of the output of the Floyd-Steinberg error diffusion algorithm illustrates a characteristic overshoot (too dark or too light) at upward and downward transitions, or steps, in the continuous tone digital image data. As used within this specification, continuous tone refers to input data that has been quantized to a larger number of discrete values than intended for the output data.
It is an object of the present invention, therefore, to be able to adjust the edges of a given digital image in a manner to improve the overall appearance of the image. It is another object of the present invention to selectively increase or decrease the amount of edge enhancement in a given digital image using a modified error diffusion algorithm for the encoding of the continuous tone input data. Further advantages of the present invention will become apparent as the following description proceeds and the features characterizing the invention will be pointed out with particularity in the claims annexed to and forming a part of this specification.